In both color and black and white photography, there exists the desire for products which exhibit increased contrast upon exposure to light and subsequent development. This desire is based upon the realization that contrast is directly related to the appearance of sharpness; and, it follows, that products which exhibit increased contrast give the visual impression of enhanced sharpness.
Traditionally, photographers have defined contrast by two methods, both of which are derived from the D-log E curve (also known as the "characteristic curve"; see James, The Theory of Photographic Properties, 4th ed. pp 501-504). The first method is the determination of gamma (.gamma.), which is defined as the slope of the straight-line section of the D-log E curve. The second is the determination of the overall sharpness of the toe section of the D-log E curve. By sharpness of the toe section, it is usually meant the relative density of the toe section. For instance, a sharp toe corresponds to a relatively low (small) toe density, and a soft toe corresponds to a relatively high (large) toe density. Generally, the point at which toe density is measured corresponds to 0.3 log E fast of the speed point, although toe density may properly be measured at any point prior to the curve's primary increase in slope. The speed point corresponds to the point on the D-log E curve where density equals 1.0.
If either the value of .gamma. is high or the toe is sharp, then the image has a relatively high contrast. If the value of .gamma. is low or the toe is soft, the image has a relatively low contrast.
It is known that in attempts to maximize the contrast of photographic elements based on silver halide emulsions (as well as other characteristics of the photographic element), the silver halide emulsions have been doped with various transition metal ions and compounds. Dopants are substances added to the emulsion during silver halide precipitation which become incorporated within the internal structure of the silver halide grains. Because they are internally incorporated, they are distinguished from substances added post-precipitation such as chemical or spectral sensitizers. These latter compounds are externally associated with the surface of the silver halide grains and are thus more properly referred to as addenda or grain surface modifiers.
Depending on the level and location of dopants, they may modify the photographic properties of the grains. When the dopants are transition metals which form a part of a coordination complex, such as a hexacoordination complex or a tetracoordination complex, the ligands can also be occluded within the grains, and they too may modify the grain's photographic properties.
Specific examples of doped silver halide emulsions can be found in U.S. Pat. No. 4,147,542, which discloses the use of iron complexes having cyanide ligands; U.S. Pat. Nos. 4,945,035 and 4,937,180 which disclose the use of hexacoordination complexes of rhenium, ruthenium and osmium with at least four cyanide ligands; and U.S. Pat. No. 4,828,962, which discloses the use of ruthenium and iridium ions to reduce high intensity reciprocity failure (HIRF).
Recently, emulsion dopants have been described which comprise transition metal complexes having nitrosyl or thionitrosyl ligands. European Patent Applications 0325235 and 0457298 disclose the use of one such complex, namely potassium ferric pentacyanonitrosyl. A second type of dopant, rhenium nitrosyl or rhenium thionitrosyl is disclosed in U.S. Pat. No. 4,835,093; and a third, dicesium pentachloronitrosyl osmate, is disclosed in U.S. Pat. No. 4,933,272.
It has also been known to use combinations of dopants in silver halide emulsions. Such combinations of dopants can be found in U.S. Pat. No. 3,901,713, which discloses the addition of both rhodium and iridium compounds during emulsification or the first ripening; and in U.S. Pat. No. 3,672,901, which teaches the combined use of iron compounds and iridium or rhodium salts.
Methods of improving the photographic characteristics of silver halide emulsions have also consisted of adding transition metals to the emulsions during chemical or spectral sensitization. As mentioned, transition metals added in this manner, because they are added subsequent to silver halide precipitation, are referred to as grain surface modifiers rather than dopants.
The most prevalent chemical sensitizers are the gold and sulfur sensitizers, both of which are thought to enhance emulsion speed by forming electron traps and/or photoholes on the silver halide crystal surface. Sensitization has also been accomplished by the addition of other transition metals. Specifically, platinum salts have been used, although sensitization with such salts is strongly retarded by gelatin. In addition, iridium salts and complex ions of rhodium, osmium, and ruthenium have been used as chemical sensitizers (and also as dopants). The overall effect of these metals on sensitivity appears to be dependent upon their valence state.
Although it is known to employ transition metals, and combinations thereof, as either dopants or grain surface modifiers, prior applications of such transition metals have yielded emulsions exhibiting inferior contrast improvement. This has often been the result of one dopant or grain surface modifier exerting an insufficient effect; or the result of a combination of dopants or grain surface modifiers exerting opposing effects.
Accordingly, it would be desirable to overcome these deficiencies by providing a high contrast silver halide emulsion exhibiting a high .gamma. and sharpened toe, wherein a combination of dopants imparts the high contrast characteristic.